The present invention relates to water heaters in general and to bottom fired central flue water heaters with internal heat exchange structures in particular.
Heat exchange between a liquid and a gas is a process which has many industrial and domestic applications. Perhaps one of the most widely used applications of heat exchange between a gas and a liquid is in heating hot water. Typically a hot water heater has a tank which holds the water to be heated, and a burner producing hot combustion gases. The water is heated by the combustion of fuel with air in the burner to produce combustion gases which heat the bottom of the tank and are vented through a centrally located flue/heat exchanger which extends through the hot water tank. Two considerations which are paramount in the design of a hot water heater are durability and efficiency. Ever since the early 1970s there has been a heightened awareness of the importance of efficiency for cost, environmental, and geopolitical reasons. Efficiency is a measure of how effectively the heat energy present in the fuel is transferred to the water contained within the hot water heater tank.
The combustion gases pass up through the central flue, exchanging heat with the wall of the flue and with the water contained within the water tank. It has long been known that internal baffles within the central flue can increase heat transfer between the flue gases and the water within the water tank. The baffles perform three basic functions. First, the baffles slow the passage of the combustion gases through the flue, giving more time for heat transfer between the gases and the flue wall. Second, the baffles mix the combustion gases within the flue, bringing more of the flue gases into contact with the flue wall which transfers heat to the water. Third, the baffles conduct heat to the wall of the flue.
As efforts are made to increase efficiency, i.e. the percentage of the combustion energy which is transferred to the hot water, at some point increased efficiency requires utilizing heat released by condensing water vapor which is produced by the hydrogen contained in the common gaseous or liquid fuels. Because the latent heat of water vapor is relatively high, approximately a thousand BTUs per pound, a relatively large amount of the energy of combustion is contained in the latent heat of evaporation of the water vapor or steam, formed as a combustion byproduct. A pound of natural gas when combusted with dry air will produce about 2¼ pounds of water, a pound of heating oil will produce approximately 1.4 pound of water. When the relative heating values of the fuels are taken into account approximately 7% of the heat of combustion of number two oil is contained in the latent heat of the water produced during combustion, and approximately 10% of the heat of combustion of natural gas is contained in the latent heat of the water produced during combustion. Therefore, a number of gas water heaters have been developed which employ heat exchangers which condense at least some of the water contained in the flue gases. Such systems have been described as having efficiencies of 90 to 96%. Condensing heat exchangers must be arranged to drain downwardly, and must be designed to overcome the corrosion potential of liquid water, which often contains small amounts of potentially corrosive contaminants from the intake air or from the combustion gases.
What is needed is a water heater which utilizes the heat transfer capabilities of a finned flue, but achieves greater efficiencies by also utilizing a condensing flue while at the same time preventing water from entering the finned flue.